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        IMA

Husky: Optimising high performance polymer moulding

Also, download this story from the electronic issue here

When moulding high-temperature polymers, direct-gating with a hot runner system can save moulders time and money, says Mohammad Rafi, BASc, Applications Engineer at Husky Injection Molding Systems.

Hot runner systems were introduced in the 1960s to mould commodity polymers such as polypropylene (PP), polyethylene (PE) and polystyrene (PS). Since that time technology innovations have improved the capabilities of hot runner systems.

The industry has evolved from hot runners direct-gating parts moulded from commodity polymers to hot runner systems producing parts from engineering polymers, such as polycarbonate (PC), polyethylene terephthalate (PET) and polyoxymethylene (POM) - also known as acetal, polyacetal and polyformaldehyde. Engineering resins are high-strength plastic materials that have superior mechanical and thermal properties over commodity plastics. They are often resistant to high temperatures, wear and corrosion.

Catering to engineering resins

For engineering or high temperature polymer applications, care must be taken to ensure that the hot runner manifold is thermally stable with :

i) no dead spots or areas where the material can sit and degrade
ii) without hot or cold spots

These resins are temperature and residence time sensitive meaning that the material will degrade if exposed to heat for extended period of time. The total runner volume in the hot runner system should not exceed a few shots for this reason.

The injection moulding machine should be sized in such a way that the total shot-weight of the parts is about 70% of the machine shot capacity, therefore the molten plastic will not remain in the barrel over numerous cycles. Due to a lack of understanding of the above mentioned criteria, cold-runner technology has been primarily used to mould parts from high-temperature polymers (see Figure 1).

F-1

Producing parts with a cold runner could be unnecessary as engineering resins are expensive and in many cases the cold runners cannot be reground and recycled. Any cold runner applications that are currently using engineering polymers could benefit from a switch to a hot runner system. Converting production from a cold runner to a hot runner and direct-gating hightemperature polymer plastic parts can yield a savings of up to 50%, which includes savings from resin waste, energy, labour and the mould.

Processing high-temperature polymers

A key development in the moulding of hightemperature polymers was the introduction of polyaryletherketones (PAEK), a family of semicrystalline thermoplastics with high-temperatures.

Polyetheretherketone (PEEK) belongs to this family and is one of the more popular engineering resins because of its thermal stability (viable at temperatures of up to 260ºC), high strength-toweight ratio, resistance to impact, chemicals and radiation and dimensional stability. This makes it an optimal choice for complex part geometries with thin cross-sections.

To help demonstrate the importance of these resin and hot runner developments, consider the processing requirements and issues that were encountered when moulding a mobile device housing (back cover). These parts were produced with a PEEK/PAEKgrade resin ideal for molding thin-wall parts with high strength and stiffness requirements.

F-2

This experimental project has a few challenges. The first was gating: direct-gating these parts with hot-tip style gates was not an option because the PEEK resin would freeze off at the normal processing temperature. Raising the temperature of the tip (at the gate) to over 340ºC could result in degrading the resin, therefore torpedo-style hot tips are not recommended for gating PEEK parts.

Other gating styles to consider include valve gates (VX Tip) and standard thermal sprue gates (TS) (see Figure 2). With these gating styles, the gate area is maintained at a higher temperature as the melt heat is brought to the gate via the nozzle/valve stem or the gate tip.

This part was direct-gated with a valve gate hot runner system. The plaque, measuring 4 in. in length and 1.5 in. in width with 1-mm wallthickness, was moulded from a single-drop offset hot runner mould in a 180-tonne Husky injection moulding machine.

The single drop offset hot runner F-3 consisted of a valve gate nozzle with a VX Tip gate style (see Figure 3). The VX gate has no material insulation area and conducts heat into the gate through the nozzle and thereby keeps it warm as needed for semicrystalline resins such as PEEK.

PEEK grades are semicrystalline thermoplastics with melt temperatures in the range of 350º-400ºC and the mould temperature in the range of 150º-200ºC. Since the processing temperatures for PEEK resins are high, the hot runner system including the heated manifold and nozzle, must be capable of maintaining high temperatures to keep the resin in molten state. Pneumatic valve-gate actuators perform well in these extreme operating conditions.

Before processing, the resin has to be properly dried at 120-150ºC for 3-5 hours. If the resin is not properly dried then splay or bubbles could form in the part, which can affect part quality.

Keep in mind that injection speed, hold time, hold pressure, shot size and processing temperatures (melt and mould) influence part and gate quality, so each of these parameters must be fully optimised to establish a wider processing window, increasing part quality and improving repeatability.

Optimising part fill times

To optimise part fill-time, several runs at different injection speeds at a set melt temperature of 385ºC and mould temperature of 170ºC were conducted. These runs started with a high injection speed of 300 mm/s and then slowed down to 25 mm/s. The results demonstrated that the part needed to be filled faster. The optimum fill time was determined to be 0.32 seconds and peak pressure was 10,954 psi F-4
(see Figure 4). High-resolution imaging of the gate demonstrated the gate quality that can be achieved with the right equipment and processing conditions (see Figures 5 and 6).

F-5-6

Gate posting (vestige) was also observed during processing with melt temperatures at 390-395ºC. F-7 Lowering the melt temperature resolved the issue. Also, ripples and grooves on the part surface were observed at low injection speeds. This was due to solidification occurring too quickly. The high resistance to resin flow produces uneven frontal flow and solidified resin will not fully contact the cavity wall. Even longer holding pressure was unable to smooth out the ripples (see Figure 7). By increasing the injection speeds, the ripples and record grooves were eliminated.

Conclusion

Although high-temperature polymers with processing temperatures in the range of 350-400ºC present certain moulding challenges, as previously mentioned, hot runners are still the optimal choice for small to medium size part applications when direct-gating is used.

Mould makers and moulders can justify the increased upfront costs for direct-gating these types of plastic parts due to the substantial cost savings over the product lifecycle.

It is crucial to select a suitable hot runner design to ensure that the entire melt channel, from the sprue bushing to the gate are maintained to within +/- 5ºC of the set temperatures to avoid any hot/cold spots for crystalline polymers.

Acknowledgements: Eric Beauregard for helping to run the resin test and Patrick Clemensen at Victrex PLC for supplying the resin for the resin test.

For more information contact: Husky Injection Molding Systems http://www.husky.co

(IMA)


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